1. Field of the Invention
The invention relates to a charging apparatus for charging a secondary battery.
2. Description of the Related Art
A charging apparatus typically charges a secondary battery, such as a lithium battery or the like, using a constant current charge followed by a constant voltage charge. FIG. 3 illustrates the charging process. When a battery voltage of the secondary battery is equal to or less than a set voltage of the charging, the battery is charged by a constant charge current and the battery voltage is raised. This portion of the charging process is depicted at the segment of FIG. 3 labeled "constant current charge." When the battery voltage reaches the set voltage, the charge current decreases and the battery is charged by a constant charge voltage, as illustrated by the segment of FIG. 3 labeled "constant voltage charge."
FIG. 1 illustrates a conventional charging apparatus P. In the charging apparatus 10, a secondary battery 11 and a current detecting resistor 12 are serially connected to a power source 13. That is, a positive terminal of the secondary battery 11 is connected to a positive terminal of the power source 13, a negative terminal of the secondary battery 11 is connected to one end of the current detecting resistor 12, and the other end of the current detecting resistor 12 is connected to a negative terminal of the power source 13. Positive and negative input terminals of a comparator 14 are connected to both ends of the current detecting resistor 12. An output terminal of the comparator 14 is connected to a charge control unit 15, which is also connected to a display unit 16. FIG. 2 illustrates the relationship between charge current and charge voltage in the charging apparatus P.
As illustrated in FIG. 3, in such a construction, at the last stage of the charging of the secondary battery 11, after the charging by a constant current Ib1 was finished, the charging by a constant voltage Vb1 is started, so that the charge current I decreases. The current detecting resistor 12 detects the charge current I, and the comparator 14 compares the detected charge current I with a current set by a reference voltage E1 and outputs a detection signal to the charge control unit 15. If the detection signal is at the high level, the charge control unit 15 determines that the charging should continue and causes the display unit 16 to indicate that the battery is being charged. On the other hand, if the detection signal is at the low level, the charge control unit 15 determines that charging is finished, and causes the display unit 16 to indicate this status.
In the conventional charging apparatus 10, the speed with which the battery is charged is limited. In particular, the constant voltage charging time is determined by an impedance in the secondary battery. As a result, the time required to charge by the constant voltage is 3t, where t is the amount of time required to charge by the constant current. Thus, the total charge time is 4t. To reduce the charging time, it is necessary to increase the charge current of the constant current charge. Still, the constant voltage charging time occupies 3/4of the charging time and cannot be reduced. Moreover, the high output (high electric power) apparatus required to charge the secondary battery using a higher constant current is relatively expensive.